Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for steering sources while being towed in water by a vessel.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which are especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon reservoirs.
A traditional system for generating the seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to a surface 14 of the ocean. The streamers may be disposed to have other than horizontal spatial arrangements. The vessel 10 also tows a seismic source array 16 that is configured to generate a seismic wave 18. The seismic wave 18 propagates downwards toward the seafloor 20 and penetrates the seafloor until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upwardly until it is detected by the receiver 11 on the streamer 12. Based on the data collected by the receiver 11, an image of the subsurface is generated by further analyses of the collected data.
The seismic source array 16 may include plural individual source elements. The individual source elements may be grouped in sub-arrays, so that each source array may include one or more sub-arrays. The individual source elements may be distributed in various patterns, e.g., circular, linear, at various depths in the water. FIG. 2 shows a vessel 40 towing two cables 42 provided at respective ends with deflectors 44. Plural lead-in cables 46 are connected to streamers 50. The plural lead-in cables 46 also connect to the vessel 40. The streamers 50 are maintained at desired separations from each other by separation ropes 48. Plural individual source elements 52 are also connected to the vessel 40 and to the lead-in cables 46 via ropes 54. However, this configuration does not allow an accurate control of the plural individual source elements. In other words, a position of the source array 16 cannot be adjusted except by changing the position of the vessel 40.
Further, the presence of the deflectors 44 introduces a further control problem as the deflectors rely on hydrodynamic forces, e.g., lift, created by the motion through the water to pull the streamers 50 outwardly to maintain their separation relative to the vessel path during the survey. Thus, water currents or other environmental factors may affect the lift, determining the deflectors to move closer to each other. As such, the positions of the streamers 50 and the plural individual source elements 52 are also affected by the positions of the deflectors. In this instance, the navigation system of the vessel modifies a trajectory of the vessel for bringing the sources as close as possible to a desired position. However, it is undesirable to use the navigation system of the vessel to adjust the positions of the sources because a change in the position of the vessel also changes the positions of the streamers.
Further, as four-dimensional (4-D) geophysical imaging is becoming more desired today, controlling the positions of the source array and of the streamers is important. 4-D geophysical imaging involves 3-D seismic surveys repeated over the same subsurface at different moments in time to determine changes in the geophysical structures of the subsurface. Thus, as the 3-D survey is repeated in time, sometimes after a few months or years, it is desirable that the sources and streamers being used to perform the seismic survey are located as closely as possible to the same locations as in the previous survey over the subsurface.
Thus, it is challenging with existing source steering technology to position both the source arrays and the streamers, at different moments in time, at the same locations given the cross-currents, wind, waves, shallow water and navigation obstacles that are currently encountered by vessels that perform the seismic surveys.
Accordingly, it would be desirable to provide systems and methods that provide a method for steering source arrays at a desired location during towing underwater by a vessel without changing the position of the vessel.